For the past few years we have been trying to unify our Atlassian Connect and Server/Data Center code base in a way that allows us to re-use as much code as possible.
This has resulted in a Atlassian Connect Polyfill library that includes both server-side and client-side code which will allow you to fully benefit from the Atlassian Javascript API (a.k.a. AP) in your Data Center environment and have all your front-end code run in isolated iframes within Data Center.
The client-side code has been available with an Apache 2.0 open source license. We are now also releasing our server-side code with the same permissive Apache 2.0 license 
The code base comes with support for Jira and Confluence, with Bamboo and Bitbucket following soon. Apart from an extended readme on how to implement the library, there are also two example Atlassian plugin projects in the code base that you can use as reference.
As always we hope that this can benefit the Atlassian community in building their apps for the Atlassian Ecosystem!